Sculpting brushes other than the grab brushes aren't affecting most of my mesh
Go to object mode, select the mesh, press Ctrl + A, then choose Scale.
here more info: https://docs.blender.org/manual/en/2.79/editors/3dview/object/editing/transform/clear_apply.html#apply-object-transformations
Why is it important to apply transformation to an objects data?
this is the problem: negative scale (and not uniform)